Parental occupational exposure to pesticides has been inconsistently associated with an increased risk of childhood leukemia. Most previous studies have been limited by the use of job titles alone to estimate pesticide exposure or lack of adequate statistical power to detect an association. In this proposal, we will evaluate and quantify the impact of exposure misclassification that results from using job titles alone compared to detailed job history information using task-based job modules, when determining the risk of childhood leukemia. We will conduct secondary analyses of data from the Northern California Childhood Leukemia Study (NCCLS), a large case-control study with more than 40% Hispanic participants and 15% of participating parents with potential occupational pesticide exposure. In addition, the NCCLS includes measurement of concentrations of pesticides in home carpet dust for a subset of participants that will be used to identify agricultural pesticides with potential for contamination of the home from parental occupational exposure. We will determine the variability of pesticide concentrations explained by job titles compared to task-based job modules, and assess exposure measurement error.